The present invention relates to a clutch of an automatic transmission vehicle (AT vehicle). AT vehicles have a problem that shock under gear shift should be reduced as much as possible because gear shift is automatically performed. The shift shock can be recognized as “the amplitude of acceleration which a driver senses bodily”. The driver's bodily-sensed acceleration amplitude occurs due to variation of a countershaft torque under gear shift (the driving force of a rear wheel as a driving wheel is also in one-to-one relation with the countershaft torque, and variation of the driving force of the rear wheel can be regarded as being the same as the variation of the countershaft torque).
The principle of occurrence of the shift shock will be described hereunder.
A condition that shift-up from 1-speed to 2-speed is performed in a conventional motorcycle as disclosed in Patent Document 1 is assumed. FIG. 18 shows an example of ratios of driving system parts. The initial value of the number of rotations of a clutch outer is set to 1500 rpm. The ratios and the initial value of the number of rotations of the clutch outer are expediencies. Particularly, various values may be taken as the initial value of the number of rotations according to the driver's driving manner.
FIG. 16 shows the transition of the countershaft torque under up-shift, and FIG. 17 shows the transition of the number of rotations of the driving system parts under up-shift. The difference in number of rotations between the clutch outer and the clutch center is defined as “clutch rotation number difference”.
First, attention is paid to the countershaft torque and the transmission of the rotation number.
(1) Before an up-shift operation, the number of rotations of the clutch outer is set to 1500 rpm in accordance with Ne-Th (Engine rotation number-Throttle opening) at that time, and a fixed countershaft torque occurs. With respect to the number of rotations of the driving system parts, the number of rotations of the clutch center would be equal to 1500 rpm if no slipping is assumed because the clutch is set under a clutch engagement state. The clutch center rotates integrally with a main shaft. The countershaft is driven by the main shaft, and the number of rotations of the countershaft is equal to 500 rpm in accordance with the gear ratio 3 of 1-speed. The rear wheel is driven by the countershaft, and the number of rotations of the rear wheel is equal to 200 rpm in accordance with the final reduction ratio of 2.5.
(2) When gear shift is started and the clutch is released, the driving force is not applied from the engine to the countershaft, so that the countershaft torque is temporarily equal to zero. With respect to the number of rotations of the driving system part, the clutch outer would be kept to 1500 rpm if it is simply assumed that Ne is kept constant because a mechanical throttle is used in the embodiment. However, under the state of (2), the clutch is released, so that the main shaft, the countershaft and the rear wheel are not driven by the engine (the rotation of the clutch outer). Under the clutch OFF state, the rear wheel side is set to drive the transmission side. Here, when it is assumed that the number of rotations of the rear wheel is fixed to 200 rpm while the vehicle speed is kept constant, the number of rotations of the countershaft is equal to 500 rpm in accordance with the final reduction ratio of 2.5, and the number of rotations of the main shaft (clutch center) is equal to 1500 rpm in accordance with the deceleration ration 3 of 1-speed. As a result, even under the clutch OFF state, the difference in the clutch rotation number is equal to zero at the time point at which the gear position is set to 1-speed.
(3) Subsequently, the gear position shifts to 2-speed while the clutch is kept to be released. Since the clutch is kept to be released, the countershaft torque is kept to be zero. With respect to the rotation number of the driving system part, the clutch outer is kept to 1500 rpm. Even under the state of (3), the rear wheel side is set to drive the transmission side. Therefore, when the number of rotations of the rear wheel is equal to 200 rpm, the rotation number of the countershaft is equal to 500 rpm in accordance with the final reduction ratio of 2.5. Here, when the gear position shifts to 2-speed, the number of rotations of the main shaft (clutch center) is equal to 750 rpm in accordance with the deceleration ratio 1.5 of 2-speed. That is, the number of rotations of the clutch center decreases to be lower than that under state of (2) under which the gear position is set to 1-speed. As a result, there occurs a difference in clutch rotation number of (clutch outer 1500 rpm)−(clutch center 750 rpm)=(clutch rotation number difference 750 rpm) at the time point when the gear position shifts to 2-speed under the clutch OFF state.
(4) When the clutch is engaged, the clutch rotation number difference is absorbed by the clutch. The multiple plate clutch is configured so that two rotators having different numbers of rotations are connected to each other in accordance with the capacity of the clutch while slipping so as to be matched with the same rotation number. When the clutch capacity is large, the difference in clutch rotation number per unit time can be greatly reduced. At this time, a shaft at which the number of rotations is increased due to the clutch engagement is instantaneously accelerated, and thus the torque of the shaft concerned greatly rises up while the clutch rotation number difference is absorbed. Conversely, when the clutch capacity is small, the clutch is liable to slip and thus the decrease of the difference in clutch rotation number per unit time lessens. At this time, the shaft at which the number of rotations increases due to the clutch engagement is gradually accelerated, so that the rising degree of the torque of the shaft concerned is low. The time for which the difference in rotation number is absorbed is long.
That is, the countershaft torque during the absorption of the rotation number difference is determined in accordance with the clutch capacity. When the clutch capacity is large, the countershaft torque is large, and when the clutch capacity is small, the countershaft torque is small.
With respect to the rotation number of the driving system part, when the clutch center is assumed to be kept at 1500 rpm, the rotation numbers of the main shaft, the countershaft and the rear wheel are also switched to be driven by the engine side (clutch center). As a result, the clutch center whose rotation number is equal to 750 rpm at the time point of (3) is accelerated to 1500 rpm in the time corresponding to the clutch capacity.
(5) When the gear shift is finished after the difference in clutch rotation number is absorbed, the gear is shifted up by one step from the just-before gear state, and thus the countershaft torque is lower than that at the time point of (1). With respect to the rotation number of the driving system part, when the rotation number of the clutch outer is kept at 1500 rpm, the rotation number of the clutch center is equal to 1500 rpm because the difference in clutch rotation number is absorbed. Since the clutch center rotates integrally with the main shaft, the rotation number of the countershaft is equal to 1000 rpm in accordance with the gear ratio 1.5 of 2-speed, and the rotation number of the rear wheel is equal to 400 rpm in accordance with the final reduction ratio of 2.5.
Here, paying attention to the rotation number of the rear wheel between (3) and (5), in the process of (4), the rotation number of the rear wheel increases from 200 rpm to 400 rpm. That is, acceleration occurs. When this acceleration is rapid, the countershaft torque quickly increases, which causes shift shock.
Next, time charts will be viewed while attention is paid to acceleration amplitude of FIG. 16. (1) There is no acceleration amplitude before shift-up because the countershaft torque is constant. (2), (3) when shift change is started and the clutch is released, the countershaft torque is equal to zero, so that the bodily-sensed acceleration swings so as to decrease. (4) the bodily-sensed acceleration during absorption of the difference in rotation number difference swings so as to follow the value which corresponds to the countershaft torque corresponding to the clutch capacity. (5) Thereafter, the bodily-sensed acceleration swings so as to follow the value corresponding to the countershaft torque after the shift-up is completed.
From the viewpoint of suppressing the shift shock, it is important that the clutch capacity during absorption of the difference in clutch rotation number is variable. The reason for this is as follows. The countershaft torque before and after gear shift is changed to be high or low in accordance with the driving state, but the countershaft torque during absorption of the difference in clutch rotation number is determined in accordance with the clutch capacity. Therefore, when the clutch capacity is fixed, (4) during absorption of the difference in rotation number and (5) the clutch engagement is completed, the acceleration amplitude may be large depending on the driving state.
When the clutch capacity is variable, in order to suppress the shift shock, the clutch capacity during absorption of the difference in rotation number may be set between the countershaft torques before and after the gear shift, whereby the countershaft torque during absorption of the difference in rotation number can be adapted to the countershaft torques before and after gear shift, and the acceleration amplitude can be suppressed as much as possible. Conversely, as the clutch capacity is farther away from the band of the countershaft torques before and after gear shift, the acceleration amplitude is more intense, and thus shift shock is more easily sensed.
The multiple plate clutch disclosed in the Patent Document 1 has a mechanism that has a clutch spring for urging a pressure plate in a clutch engagement direction and a release spring for urging the pressure plate in a clutch release direction in accordance with a lift mount of a lifter plate which is displaceable relatively to the pressure plate, and in which the clutch capacity is reduced in accordance with the lift amount of the lifter plate. According to the construction of the Patent Document 1, by properly controlling the lift amount of the lifter plate when the difference in rotation between the countershaft and the engine side is absorbed at the start of the clutch engagement under gear shift, the clutch capacity can be adjusted, and the shift shock can be reduced.